applied sciences Article Strength Characteristics and Slope Stability Analysis of Expansive Soil with Filled Fissures Zhangjun Dai 1,* , Jianhua Guo 1,2, Hongming Luo 1, Jian Li 1 and Shanxiong Chen 1 1 State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan 430071, China; [email protected] (J.G.); [email protected] (H.L.); [email protected] (J.L.); [email protected] (S.C.) 2 University of Chinese Academy of Sciences, Beijing 100049, China * Correspondence: [email protected]; Tel.: +86-13667217486 Received: 4 June 2020; Accepted: 1 July 2020; Published: 3 July 2020 Abstract: Fissured expansive soils were widely distributed in the South-to-North Water Transfer Project. Most of the fissures were filled with clay, which controlled the stability of the slope. With the method of layered filling—bevel cutting—refilling and a modular design idea, the sample with a filled fissure preparation device for triaxial test was designed. After setting the filled fissures of gray-green clay in the expansive soil, triaxial tests were carried out for the samples with no filled fissures and with filled fissures with inclination angles of 15◦, 30◦, and 45◦. Then, considering the spatial distribution and the strength of the filled fissures in the slope, the stability analysis method for the expansive soil slope with filled fissures was proposed. The stability of a typical slope in Nanyang was analyzed. The results show that the c of expansive soil with filled fissures was about 12 to 15 kPa and the ' was 3◦ to 6◦. Filled fissures had an attenuation effect on the strength of the expansive soil. The larger the inclination of filled fissures, the more significant the effect of soil strength attenuation. The fissured slope stability was controlled by the filled fissures. The sliding surface was affected by the vertical fissures on the top of the slope and the slow-inclined long-large fissures in the slope, and the shape of the sliding surface was a broken line, which was basically consistent with the actual landslide. Keywords: expansive soil; filled fissure; triaxial test; sample preparation device; strength characteristics; slope stability; sliding surface 1. Introduction In expansive soil slope engineering, it has always been known that it will slide every time after cutting [1–3]. The mid-route of the South-to-North Water Transfer Project in China has a total length of over 1472 km, among which expansive soil areas will cover about 340 km. Expansive soils are mainly Middle Pleistocene alluvial and flood sedimentary clay, Lower Pleistocene flood sedimentary clay, and Neogene hard clay. Strong expansive soil is mainly distributed in the Xingtai-Handan area and Nanyang Basin. The instability of the expansive soil slope will seriously affect engineering construction and safe operation [4–7]. Fissures are one of the three major characteristics of expansive soil, and they are also the most intuitive and obvious feature in expansive soil engineering. The development of fissures complicates the strength characteristics of expansive soil [8–10]. The strength of the expansive soil is comprehensively controlled by the distribution, density, tendency, inclination, extension, and filling of the fissures, and the directional arrangement of the fissures causes the anisotropy of the strength of the expansive soil [11–13]. Due to the significant effect of fissures on the stability of expansive soil slopes, research on fissures in expansive soil has been carried out repeatedly [14–17]. A digital image acquisition system was Appl. Sci. 2020, 10, 4616; doi:10.3390/app10134616 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, x FOR PEER REVIEW 2 of 17 Appl. Sci. 2020, 10, 4616 2 of 16 Due to the significant effect of fissures on the stability of expansive soil slopes, research on fissures in expansive soil has been carried out repeatedly [14–17]. A digital image acquisition system employed to capture the evolution and propagation of fissures in the soil specimen subjected to was employed to capture the evolution and propagation of fissures in the soil specimen subjected to desiccation [18]. The effect of the drying environment, i.e., temperature and relative humidity, on the desiccation [18]. The effect of the drying environment, i.e., temperature and relative humidity, on the engineering properties and the development of fissures of an expansive soil was investigated [19]. engineering properties and the development of fissures of an expansive soil was investigated [19]. Suction measurements, free shrinkage tests, constrained shrinkage tests, and splitting tensile strength Suction measurements, free shrinkage tests, constrained shrinkage tests, and splitting tensile strength tests (STTs) were undertaken to investigate the shrinkage cracks in expansive soils [20]. The influential tests (STTs) were undertaken to investigate the shrinkage cracks in expansive soils [20]. The factors such as soil matric suction on the formation of surficial fissures and the stability of unsaturated influential factors such as soil matric suction on the formation of surficial fissures and the stability of soil slopes were studied [21]. The researchers divided the strength of expansive soil into the soil strength unsaturated soil slopes were studied [21]. The researchers divided the strength of expansive soil into and the fissure surface strength, carried out direct shear and triaxial tests [22–24], and introduced the soil strength and the fissure surface strength, carried out direct shear and triaxial tests [22–24], CT scanning [23], image recognition and other methods [24] to quantitatively analyze the effect of and introduced CT scanning [23], image recognition and other methods [24] to quantitatively analyze fissures on the strength of expansive soil. These studies obtained some useful conclusions. Expansive the effect of fissures on the strength of expansive soil. These studies obtained some useful conclusions. soils swell in the presence of the water and shrink in its absence, thereby producing fissures which Expansive soils swell in the presence of the water and shrink in its absence, thereby producing significantly alter their mechanical and hydraulic performance [18]. Fissures decrease the soil’s strength fissures which significantly alter their mechanical and hydraulic performance [18]. Fissures decrease and increase its hydraulic conductivity and compressibility [20]. However, there are still deficiencies. the soil’s strength and increase its hydraulic conductivity and compressibility [20]. However, there Most studies have not systematically classified and studied the fissure itself. Fissures in different sizes are still deficiencies. Most studies have not systematically classified and studied the fissure itself. and states in the expansive soil have different effects on the engineering properties of expansive soil. If Fissures in different sizes and states in the expansive soil have different effects on the engineering the fissures contain fillers, the engineering properties of the fillers also have a great influence on the properties of expansive soil. If the fissures contain fillers, the engineering properties of the fillers also strength of the expansive soil. The shape of the fissures in the expansive soil is random, and the shear have a great influence on the strength of the expansive soil. The shape of the fissures in the expansive plane in the direct shear test is difficult to extend completely along the fissure surface. In addition, soil is random, and the shear plane in the direct shear test is difficult to extend completely along the when using undisturbed soil to make triaxial samples, the fissures are easily disturbed, and the shape fissure surface. In addition, when using undisturbed soil to make triaxial samples, the fissures are of the fissures is not easy to control. easily disturbed, and the shape of the fissures is not easy to control. The limit equilibrium method [25,26] and finite element method [27] are commonly used to The limit equilibrium method [25,26] and finite element method [27] are commonly used to analyze the stability of unsaturated soil slope, which are suitable for the analysis of the stability analyze the stability of unsaturated soil slope, which are suitable for the analysis of the stability of expansiveof expansive soil slopes. soil slopes. Numerical Numerical analysis analysis using the using finite the element finite element method methodwas conducted was conducted to analyze to analyze frequent shallow slope failures on highway embankments of expansive soil in the North Texas frequent shallow slope failures on highway embankments of expansive soil in the North Texas region [2region8]. The [28 finite]. The element finite elementmethod methodwas used was to usedstudy to the study effect the of e rainfallffect of rainfallon the factor on the of factor safety of of safety fill slopesof fill slopesconstructed constructed with expansive with expansive Yazoo Yazooclay soil clay in soilMississippi in Mississippi [29]. A [29 numerical]. A numerical exercise exercise was performedwas performed on a slope on a in slope Regina, in Regina, Canada, Canada, using a using program a program that implemented that implemented the constitutive the constitutive model intomodel infinite into infiniteslope formulation slope formulation to reflect to reflect the influence the influence of the of thestress stress regime regime change change and and associated associated softeningsoftening on on unsaturated unsaturated expansive expansive